This invention relates generally to the field of measuring an analyte in a liquid medium. More specifically, it relates to an immunoassay for the measurement of an analyte in a biological sample.
Boguslaski, R. C. et al., U.S. Pat. No. 4,134,792 (1979) describe the use of a reversibly binding enzyme modulator as a labeling substance for the detection of an analyte in a liquid medium, and in particular, the use of competitive inhibitors coupled to an analog of the analyte in immunoassays.
Dorn, A. R., U.S. Ser. No. 09,328,741 filed Jun. 9, 1999 describes a method for the enzymatic measurement of mycophenolic acid in a biological sample.
Inosine-5′-monophosphate dehydrogenase (EC 1.1.1.205) catalyzes the NAD-dependent oxidation of inosine-5′-monophosphate (IMP) to xanthosine-5′-monophosphate (XMP), Magasanik, B. et al., J. Biol. Chem. 226:339-350 (1957) and Jackson et al., Nature 256:331-333 (1975). The enzyme follows an ordered Bi—Bi reaction sequence of substrate and cofactor binding and product release. First, IMP binds to IMPDH. This is followed by the binding of the cofactor NAD. The reduced cofactor, NADH, is then released from the product, followed by the product, XMP. This mechanism differs from that of most other known NAD-dependent dehydrogenases, which have either a random order of substrate addition or require NAD to bind before the substrate.
Two isoforms of human IMPDH, designated type I and type II, have been identified and sequenced, Collart et al., J. Biol. Chem. 263:15769-15772 (1988) and Natsumeda et al., J. Biol. Chem. 265:5292-5295 (1990). Each isoform is 314 amino acids, and both isoforms share 84% sequence identity. Both IMPDH type I and type II form active tetramers in solution, with subunit molecular weights of 56 kDa, Yamada et al., Biochemistry 27:2737-2745 (1988).